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α-MoO₃ is known to support highly confined and in-plane anisotropic phonon polaritons (PhPs). In the mid-IR regime, there exist three Reststrahlen bands (RB) for PhP generation, where the α-MoO₃ crystal exhibits negative permittivity along three principal axes. Compared to boron nitride and graphene, PhPs in α-MoO₃ have remarkably low loss that can lead to practical applications in waveguiding. In addition, PhPs in α-MoO₃ can be engineered by multiple factors, such as twisting the angle between adjacent layers and controlling the surrounding dielectric environment.
To investigate the impact of the dielectric environment pn PhP propagation, the authors used Bruker's nanoIR3-s with s‑SNOM to study thin flakes of α-MoO₃ placed on a SiO₂/Si substrate with prepatterned submicron trenches. The area of α-MoO₃ covering the trench is freestanding, while the rest is supported by the substrate.
The findings from this work provide the foundation for guiding high-Q PhPs in α-MoO₃ at desired directions that can be potentially used for future nanophotonic and polaritonic devices.
To further demonstrate such impact, a circularly freestanding α-MoO₃ channel with submicron width was created and studied. s-SNOM imaging results showed that PhPs propagated along the curved trajectory.
*by varying the angle (θ) of the trench relative to the  direction